Optical reflectometric methods are widely employed for characterization of optical fiber links. Among these methods, the most commonly used approach is Optical Time-Domain Reflectometry (OTDR—also used to refer to the corresponding device), a diagnostic technique where light pulses are launched in an optical fiber link and the returning light, arising from backscattering and reflections along the fiber link, is detected and analyzed. Various “events” along the fiber link can be detected and characterized through a proper analysis of the returning light in the time domain.
Most OTDRs provide an automatic mode, where the instrument automatically chooses an appropriate pulse duration (also referred to as “pulsewidth”), acquisition range and averaging time. In order to choose the appropriate settings for the final acquisition, the instrument launches one or many short “investigation acquisitions”, which provide a quick overview of the link being tested. In general, the investigation acquisitions are hidden from the user, and only results from the final acquisition is made available.
For both manual settings and automatic settings, the final result is an OTDR trace performed with pulses having a common duration. In general, a given pulsewidth will be selected to allow characterization of the complete link. For example, a link having large loss requires testing with a long pulse. However, the use of a long pulse brings certain limitations in the ability to characterize short fiber sections, as well as closely spaced events.
An improvement to the single-pulsewidth approach has been developed, whereby the equipment makes use of successive acquisitions performed with increasingly longer pulses. Such an approach is the basis of the IntelliTrace Plus™ technology by Tektronix (http://www.tek.com, see also U.S. Pat. No. 5,155,439 (HOLMBO et al.) and U.S. Pat. No. 5,528,356 (HARCOURT)). The link under test is virtually split into segments, wherein each segment of the link is characterized using acquisition(s) performed with a given pulsewidth. Shorter pulses are used to characterize the most proximal segment, i.e. the near end of the link under test. A segment of the optical link that is farther away is then characterized using longer pulses. This acquisition process is repeated, each time using a different, generally longer, pulsewidth, until the end of the link is identified. It is to be noted that the number of different pulsewidths that are required depends on the link under test (normally only one pulsewidth for a short link, many pulsewidths for a long link). The pulsewidths of the successive acquisitions can be chosen in a dynamic manner, or using a “fixed recipe”, that is, always testing with a given sequence of pulsewidths.
A difficulty arises when the results of such a multiple-pulsewidth acquisitions are to be graphically represented. Because most operators are accustomed to rely on an OTDR trace to interpret OTDR measurements and perform diagnosis, the display of a plurality of OTDR traces—obtained under different conditions—may give rise to confusion. Attempts have been made to solve this issue by combining the information obtained from the acquisitions taken under different pulsewidth conditions to yield a single result, wherein selected segments of the multiple OTDR traces are stitched together to construct a single synthetic OTDR trace (see U.S. Pat. No. 5,528,356 (HARCOURT)). Generation of this single synthetic OTDR trace requires processing of the multiple acquired OTDR traces.
The above multiple-pulsewidth approach offers significant improvement to the traditional single-pulsewidth approach, as each event can be characterized by an “optimum” pulsewidth. However, certain drawbacks remain. For example, the optimum pulse for measuring loss is not necessarily the same as the optimum pulse to measure reflectance or to perform event location. Moreover, situations exist where a single pulsewidth is insufficient to characterize an event.
A further improvement to the multiple-acquisition approach has been developed by EXFO and is commercialized as the Intelligent Optical Link Mapper (iOLM) (http://www.exfo.com, see also U.S. Pat. No. 8,576,389 (PERRON et al), commonly owned by the Applicant). This technology also uses acquisitions performed with pulses of various pulsewidths, but, in this case, the characterization of a single event may employ a combination of a plurality of acquisitions performed with different pulsewidths. For example, a first acquisition performed with one pulsewidth may be used to characterize the event location and a second acquisition performed with another pulsewidth may be used to characterize the insertion loss or the reflectance associated with this same event. Using this approach, the link under test generally cannot be split into aforementioned virtual segments associated with respective pulsewidths and it is not possible to construct a unique synthetic OTDR trace to graphically represent the OTDR measurement. Again, suitable graphical representation of the results is problematic.
Accordingly, the iOLM approach makes use of a table of events listing the events identified in the link under test with their respective parameters, as characterized in accordance with iOLM technology. A block diagram is also displayed showing a series of blocks each representing an event. This block diagram provides a simple global view of the link, and circumvents the need for the operator to carry out a complex interpretation of multiple OTDR traces. The operator clicks on the block corresponding to an event in order to have access to its respective parameters as characterized. However, using this approach, none of the OTDR traces could be used to graphically represent all the information that is being extracted from the iOLM analysis. In the context of link diagnosis, this approach lacks a proportionally scaled graphical representation of the backscattering/reflection level along the link under test.
In the case of OTDR measurements made with single-pulsewidth acquisitions, the interpretation of conventional OTDR traces typically displayed to the operator is complicated, requiring the operator to possess a level of skill more typical of a qualified technician. As OTDR measurements become more widely used in the optical telecommunication industry, the level of qualification of OTDR operators is likely to decline, rendering misinterpretations and wrong diagnoses more frequent and consequently increasing the time spent for completing service calls.
There is therefore a need for an improved processing of data obtained from OTDR measurements in general, and more specifically of multiple-acquisition OTDR measurements employing multiple pulsewidths.